DC8 CARGO FLC CLBS ABOVE ASSIGNED ALT OF 5000 FT WITH A POLICE ACFT AT 6000 FT. ALTDEV.

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BREAKDOWN OF SEPARATION FOLLOWING DEP FROM SYDNEY INTL ARPT; SYDNEY; AUSTRALIA; JUN/XX/96. FOLLOWING DEP FROM RWY 34L AT KINGSFORD-SMITH (SYDNEY INTL) WE WERE CLRED TO CLB AND MAINTAIN 5000 FT. DURING CLBOUT WE ACCOMPLISHED NORMAL CHKLIST DUTIES WITHOUT INCIDENT. WHILE PERFORMING MY POST DEP DUTIES AT THE FLT ENGINEER PANEL THE CTLR CONTACTED OUR ACFT CONCERNING ALTDEVS. AS I LOOKED FORWARD TO SEE WHAT ALT WE WERE CLBING THROUGH ONE OF THE PLTS HAD ALREADY STARTED AN AGGRESSIVE CHANGE OF ACFT ATTITUDE. DURING THIS TIME THE MAX ALT I OBSERVED WAS 5800 FT MSL ON THE CAPT'S ALTIMETER. THE CTLR RPTED TFC AT OUR 10 O'CLOCK POS AND ASKED IF WE HAD THE TFC IN SIGHT. THE CAPT ACKNOWLEDGED THE TFC AND WE WERE TOLD TO DSND TO 5000 FT. WHILE DSNDING TO 5000 FT THE CTLR TOLD US TO LEVEL OFF AT 5500 FT. A SHORT TIME LATER WE RECEIVED CLRNC TO CLB. CONTRIBUTING FACTORS: AS WE APCHED OUR ASSIGNED ALT SEVERAL THINGS WERE HAPPENING. THE CTLR CONTACTED OUR ACFT WITH INFO REQUIRING THE PLTS TO UPDATE THEIR NAV INSTS AND RADIOS. THE ACFT WAS BEING TRIMMED BY THE PF THE DEP; RESULTING IN SEVERAL LONGITUDINAL TRIM HORN ACTIVATIONS. AS PREVIOUSLY MENTIONED; I WAS ENGAGED AT THE ENGINEER'S PANEL. DURING THIS TIME THE ALT WARNING HORN ACTIVATED. MY PERCEPTIONS; JUDGEMENTS; DECISIONS: IT IS MY BELIEF THAT I INTERPRETED THE ALT WARNING HORN AS ANOTHER ACTIVATION OF THE TRIM HORN (THOUGH THE 2 HAVE VERY DIFFERENT SOUNDS) AND IGNORED IT. IN ADDITION; WITH THE AMENDED CLRNC AND THE REPROGRAMMING OF THE NAV SYS I ASSUMED WE WERE CLBING TO A HIGHER ALT AND WAS NOT MONITORING OUR ALT AS DILIGENTLY AS I WOULD HAVE; HAD I BELIEVED WE WERE STILL LEVELING AT 5000 FT MSL. FINALLY; OUR RATE OF CLB WAS VERY GOOD GIVEN THE LIGHT WT OF THE ACFT AND FAVORABLE ATMOSPHERIC CONDITIONS.